The present invention relates to a temperature responsive electrical switching device and to a method of calibrating such a device. More particularly, the present invention relates to an electrical switching device including a bimetal blade which, through an actuator, causes a movable switch contact blade to be moved out of electrical contact with a stationary switch contact blade when the switching device is heated to a temperature exceeding a predetermined threshold temperature.
One type of known prior art temperature responsive electrical switching device includes a bimetal blade having mounted thereon a non-conductive actuator of fixed dimension. The actuator is moved by the bimetal blade into contact with a movable contact blade when a predetermined switch threshold temperature is reached. The movable contact blade is thereafter moved out of electrical contact with a stationary contact blade.
In such prior art switching devices, a non-conductive calibrating screw is threaded through an opening in the switch mounting structure and contacts the stationary contact blade. By rotating the non-conductive calibrating screw, the position of the stationary contact blade may be adjusted slightly. When the switching device is to be calibrated, it is connected electrically to a continuity testing device and placed in an oven which is maintained at a precise predetermined temperature. After being fully heated, the bimetal blade deflects the movable contact blade by means of the non-conductive actuator to a selected position. The adjusting screw is then utilized to move the stationary contact blade until the electrical connection between the movable and stationary contact blades is broken. Thereafter, electrical connection between the stationary and movable contact blades is provided until the predetermined temperature is reached, at which time the non-conductive actuator is moved sufficiently by the bimetal blade to contact the movable blade and move it out of the electrical contact with the stationary contact blade. The adjusting screw is fixed in its adjusted calibrated position by means of an epoxy material which is deposited around the head of the screw and thereafter cured.
It will be appreciated that such a prior art temperature responsive electrical switching device has substantial disadvantages. Manual calibration of each device is required, which may result in a substantial percentage of the switches being inaccurately calibrated. This likelihood of miscalibration is accentuated by the fact that the calibration operation must be performed in an oven. Typically, a small opening is provided in the oven wall through which a screwdriver is inserted to adjust the calibration screw. Additionally, this calibration technique results in a relatively expensive switch due to the labor costs involved.
One approach which has been taken to reduce these difficulties in production and calibration of thermal responsive switching devices which include bimetal switch elements is disclosed in U.S. Pat. No. 3,230,607, issued Jan. 25, 1966, to Gelzer. The Gelzer patent discloses a switch in which a bimetal blade is connected to one of a pair of electrically conductive posts extending through a non-conductive mounting structure. The bimetal blade cooperates with a stationary contact on the other of said pair of posts to provide an electrical circuit between the posts until a predetermined threshold temperature is reached. The posts are potted into the non-conductive mounting structure by means of a thermal curable resin. Uncured resin is deposited in a cavity in the mounting structure and the thermal responsive switch is then placed in an oven. The post bearing the stationary contact is shifted by the bimetal element as the element is heated and deflects. After the post is appropriately positioned by the bimetal element, the heat curable resin is cured, fixing the pair of posts in position in the non-conductive mounting structure. While providing a self-calibrating switch, the Gelzer calibration technique is directly applicable only to a switch structure in which the bimetal blade forms a part of the electrical circuit.
It is seen, therefore, that a need exists for an electrical switching device which is simple in construction and which may be calibrated without the need for manual adjustment of switch parts by a technician.